Consumer markets continue to request smaller, portable electronic devices having more functional features. Examples of such devices include two-way and broadcast radio receivers, compact disc players, cellular telephones, and computer devices, to name but a few. As portable electronic devices have become smaller, the demand for smaller energy sources, including batteries, has increased. While very small energy storage devices, such as an electrochemical battery cells may be fabricated for a given electrical device; compactness comes at the cost of energy capacity. Accordingly, for many high power applications the energy source is too bulky, too heavy, or doesn't last long enough.
As the energy storage device, such as a battery, is discharged, it becomes unable to provide current at a required level. Thus, even though the battery may retain a substantial charge, it is useless to the device to which it is attached. This problem is exacerbated when the device to which the battery is attached requires high power (i.e., current pulses) in an operating cycle which otherwise requires a much lower operating current. Such is the case with portable communications devices, such as digital twoway radios and cellular phones when in the transmit mode. These power pulses or spikes require significantly higher current outputs than when the device is receiving or in standby mode.
As the physical size of batteries decreases (to meet size requirements of product designers), the capacity of the battery is reduced. This results in device users needing many batteries if they anticipate being away from a battery charging device for extended periods of time. Alternatively, users may carry portable, high speed, charging devices with them. This however is unacceptable, due to the additional weight associated with the charging device.
Prior art attempts to address the high power spikes entailed providing electrolytic capacitors in the application device. This had the disadvantage of increasing substantially the size of the application device, as electrolytic capacitors are typically very large, cylindrical devices. Other attempts are described in U.S. Pat. No. 5,439,756 to Anani, et al, in which an electrical energy storage device is provided. The device disclosed in the '756 patent includes a battery electrode, a capacitor electrode, and a third electrode as the counter electrode for both the battery and the capacitor electrodes. The device also includes electronics to switch the third electrode between the battery electrode and the capacitor electrode. A second solution to this problem is provided in the aforementioned '517 application which describes a power source having a first component, for example, a battery, for delivering a substantially constant output, and a second component, a capacitor for example, which delivers power in response to the power pulses and spikes required by the application device. The capacitor can then be recharged by the battery, assuming the duty cycle permits. While this type of power source addresses the needs of pulsed power application devices, it does not address the fact that changing conditions may require a different trigger point at which the second component is activated. These conditions may be environmental, such as low temperature, or a function of the age of the first power source. Failure to recognize the effect of changed condition may also have the deleterious effect of shrinking the life of the power source. However, recharging the capacitor via the battery has the deleterious effect of imposing a power spike on the battery at the start of the capacitor recharge cycle. While this spike is not as large as that demanded by an application device, the effect on the battery, over time, can be as great. More particularly, the battery recharges the capacitor between pulses via a constant voltage mode charge. However, this mode can result in incomplete charging of the capacitor because as the voltage difference between the battery and the capacitor narrows, the current flow from the battery reduces, thus requiring a very long time to fully charge the capacitor. Thus, recharging in that mode can result in the capacitor being less than fully charged.
Accordingly, what is needed is an energy source which is capable of providing sufficient voltage for the high power pulses required of certain devices, while extending the usable life of the energy source. The device should also include means by which current drain from the battery is as smooth and uniform as possible to extend battery life. Such a device should be relatively small, and capable of being easily sized and shaped for a given application. Moreover, such a device should include a mechanism to control and/or manage recharging of the capacitor to assess a full recharge.